This invention relates to lockstitch sewing machines having mechanism for replenishing bobbin thread while the bobbin remains in place in the loop taker, and more particularly, to a system for automatically initiating bobbin winding at the end of the thread supply in the bobbin, and automatically terminating bobbin winding to revert to a sewing mode when the bobbin has been filled.
It is known in the prior art to provide bobbin thread replenishing mechanisms which are operative on an instruction from an operator to replace thread in the bobbin of a lockstitch looptaker. Such a device is shown in the U.S. Pat. No. 3,693,566, issued on Sept. 26, 1972 to Ketterer.
It is also known in the prior art to provide for a sewing machine having two bobbins situated in a bobbin case supported in a looptaker, and a sensing means to determine a low condition of a bobbin for the purpose of initiating the filling of the other bobbin utilizing a special bobbin winding motor. In this prior art device, a great deal of complication and extra parts are necessary in order to provide two bobbins, sensing means and winding means including a separate winding motor. Such a device is shown in the U.S. Pat. No. 3,332,381, issued on July 25, 1967 to Dobner et al.
There also exists in the prior art devices for detecting the depletion of thread, such as is disclosed in U.S. Pat. No. 3,129,680, of Doerner. Many or all of the thread depletion detectors in the prior art detect a condition of low bobbin thread as opposed to a condition of no bobbin thread remaining on the spool. Thus, these depletion detectors are effective to warn an operator that the bobbin thread is about to be used up. However, an automatic winding system would require that the bobbin be completely empty prior to attempting to refill the bobbin in order to avoid damaging the work material by rewinding the bobbin while a length of thread thereon remains connected to the work material.
What is required is a device to sense the complete exhaustion of the bobbin thread, to inhibit further stitching by the sewing machine, to initiate rewinding of the bobbin, and to re-initiate stitching by the sewing machine upon completion of the winding of the bobbin.